1. Field of the Invention
The present invention relates to a vehicle speed control system, and more particularly, to a vehicle speed control system capable of easily changing control constants associated with a vehicle speed control function of the system.
2. Description of the Related Art
A vehicle speed control system provided with electronic control units for vehicle speed control and engine control that cooperate with each other to make speed control is known. This control system may be configured to perform auto-cruise control for achieving a constant-speed running of a vehicle. In such a vehicle speed control system, the electronic control unit for vehicle speed control hereinafter referred to as auto-cruise Ecu) is designed to determine whether or not conditions for vehicle speed control are fulfilled, and transmits pieces of control information, such as target vehicle speed and actual vehicle speed, to the electronic control unit for engine control (hereinafter referred to as engine ECU) when it determines fulfillment of the vehicle speed control conditions. The engine ECU operates to subject the pieces of control information received from the auto-cruise ECU to appropriate processing in term of Proportional-Integral-Derivative (PID) control or the like to thereby determine operated variables (e.g., amounts of operation of an actuator for fuel injection amount control and an auxiliary braking system). In accordance with the operated variables, the engine ECU controls the operations of the actuator and braking system to control engine output so that the actual vehicle speed coincides with the target vehicle speed.
During the development of a vehicle and/or engine, their/its specifications are frequently changed, and accordingly, control functions of vehicle control systems mounted thereon are frequently adjusted in order to meet the specifications of the vehicle and/or engine. In general, an electronic control unit of a vehicle control system is provided with control logics (constituted by hardware logics and/or software logics, for instance) that determine control procedure for achieving a control function, and control constants (including PID control gains, maps, tables, determination values, for instance) for execution of the control procedure. The control function, determined by the control logics and the control constants, can be adjusted by changing the control constants associated therewith.
However, in the above-mentioned vehicle speed control system, control constants associated with determination of vehicle speed control conditions are provided in the auto-cruise ECU, whereas control constants associated with determination of operated variables and engine control are provided in the engine ECU. For this reason, in order to adjust the vehicle speed control function of the system, it is necessary to individually change the control constants provided in the auto-cruise ECU and the control constants provided in the engine ECU, resulting in a complicated changing operation. Such a complicated operation for changing the control constants poses a problem that the efficiency of vehicle and/or engine development is lowered.